The mere mention of Matt Harvey made Terry Collins sigh, a deep exhale of a sigh that perfectly encapsulated the state of the team’s one-time ace.

Harvey, still in the rotation as his ERA has ballooned to 6.08 — the fifth-worst ERA of 101 qualified starting pitchers — pitches on Monday against the White Sox, and his ongoing struggles, from lockdown dominant No. 1 starter to enigmatic No. 5, has befuddled the Mets. When asked what he expects from Harvey, Collins paused, took that deep sigh and said: “That’s a good question.”

Clearly, nobody has an answer for why the man formerly known by the superhero nickname “Dark Knight” is pitching so badly, why he hasn’t gotten past the sixth inning yet, why opponents are batting an astronomical .509 the third time through the order, and why his May has been so difficult, 1-4 with a 7.56 ERA in five starts. Is it mechanics — Ken Rosenthal of Fox Sports reported on Saturday the Mets believe they have found a flaw in Harvey’s delivery in which he pauses at one point and loses command as a result — fatigue from throwing a career-high 216 innings last year in his first season following Tommy John surgery or just a deep slump?

The Mets believe keeping him in the rotation to work through his problems is the best method at this point.

“I’m hoping that he relaxes,” Collins said. “I’m hoping he just goes out and pitches like he knows how. ‘Hey, look, worry about making pitches, not so much about the mechanics.’ It’s like anything else: If your mind’s on anything else besides throwing the baseball, if you’re worried about where your foot’s landing, you’re worried about where your arm angle is, you’re going to have a tough time.

“That’s why I hope he just goes out and pitches like he knows how. And has a little more fun than he’s been having.”

Collins declined to say how long Harvey’s leash is, whether another ugly outing could force the Mets to make a change, either to send him down or give him a breather.

“All I know is he’s pitching [Monday],” Collins said. He later added, when pressed: “We have to wring the rag dry here. … I think we need to push it a little bit farther.”

The manager is certain — and he’s said this frequently — that Harvey will eventually find himself. That he will become the “Dark Knight” again rather than the “Joker,” find the stuff and velocity that made him such a big star in the first place, go back to putting up double-digit strikeouts and pitching deep into games. But like everyone else, he just doesn’t know when that will happen.

“I believe as we continue, Matt’s gonna get better,” Collins said. “There’s no indication — if there was something we were concerned with physical, I’d say, yeah, he’s having a tough time. I think he’s going to bounce back.

“A year ago, we were all concerned about his velocity wasn’t there, his slider wasn’t there and by mid-summer, it was there. And I still think the same thing’s gonna happen this year. I still think we’re gonna look up here in weeks and it’s gonna be the Matt Harvey we know.”